1. Field of the Invention
The present invention relates to downhole tools and particularly, though not exclusively, to a torque limiting device used to allow relative rotation between two tubulars when a predetermined torque is applied between the tubulars, thereby reducing or preventing torsional damage to the tubulars or the connection between them at torque loads above the threshold of the predetermined torque.
2. Description of the Related Art
Wellbores typically contain various strings of tubulars connected end to end in the well. From time to time, for example, after drilling and casing a well, the inner surface of a tubular string such as a casing string is conveniently cleaned with a cleaning string to remove rust and other debris from the well bore. When a cleaning string is rotated by the powerful rig top drive to scrub the inner surface of the casing string, an operator must be careful to avoid applying too much torque to the cleaning string because it is common for high torque levels that can be applied by the top drive to damage the cleaning string, requiring costly intervention to repair or recover the damaged string.
The actual torque applied to each part of the string can be affected by the properties of the work string itself and by the properties of fluid that locally surround that part. Additional problems can arise in well bore cleaning operations because of the practice of circulating different cleaning fluids such as brine through the string at the bottom of the well in order to displace drilling mud or other lubricant from the annular space between the casing string and the cleaning string. As a result, different parts of the string can be surrounded locally by different fluids, and as a result, the two parts can be exposed to different frictional forces that resist the string's rotation to different extents. For example, strings rotating in brine have approximately twice the drag as compared with those run in mud and the method of pumping the brine down through the inner work string out through an aperture at the bottom of the work string and then up the annulus outside the work string to displace the mud ahead of it means that the lowermost section of a work string that is being used to pump brine may be rotating in brine whereas the uppermost might be rotating in low friction drilling mud. Such a string will therefore experience differential drag along its length. Accordingly, the torque experienced by the lower section of the work string can be different from the torque experienced at the upper section. The moving interface between the brine and the drilling mud as the mud is displaced upwards along the annulus results in additional complications for the operator, and the risks of exceeding the mechanical properties of the lower string are increased as there is no mechanism to monitor from the surface the torque that is experienced by the lower section of cleaning string in these variable conditions.
Furthermore tubular strings are often arranged in sequential sections of gradually decreasing diameter, and adjacent sections of tubular typically create steps in the diameter of the string, for example, in a work string, an upper section can often have a larger outer diameter than a lower section. Also, a casing string can have steps in the inner diameter, for example where a liner is hung inside casing. Difficulties arise in satisfactory removal of debris from e.g. casing strings with a non-uniform inner diameter because there is frequently a large annular space between some portions of the work string bearing cleaning tools and the upper sections of the casing string being cleaned. Cleaning strings used to carry out this cleaning task are therefore typically made up with at least two sequential sections of decreasing diameter so that the larger diameter upper sections of casing string are cleaned by a larger diameter section of the work string. Accordingly the cleaning string can typically have at least one step (or crossover) in its diameter and the relative mechanical properties of upper and lower sections of the cleaning string can therefore be very different. This increases the risks of torsional damage to the crossover or the weaker narrow diameter pipe below it.
In a well with 4 to 5,000 feet of liner, when the brine is circulated to the top of the liner, the rotating tubular string will experience an increase in torque as described above. To avoid damage to the lower string typically a circulation tool with a clutch device is located in the string and positioned at the liner top. The clutch device allows the lower string to be selectively mechanically disengaged from the upper string. While the clutch is engaged, rotation of the upper string is transmitted across the clutch and the upper and lower strings rotate together, but when the clutch is disengaged, the upper string can freely rotate while the lower string remains stationary. The circulation tool and clutch tend to be interdependent or even incorporated into a single tool. When the circulating brine reaches the liner top, the clutch is engaged and the circulation tool is activated to increase the pump rate of brine into the wider casing. However, when the circulation tool is ready to be activated and the clutch engaged, the relatively narrow liner annulus is already full of brine and maximum torque is already applied to the narrow section of the cleaning string below the circulation tool. Currently, in an attempt to prevent damage to the string from the torque, an operator will stop rotation of the string early. This compromises cleaning of the liner as mud may be left at the liner top. Alternatively if rotation is not stopped sufficiently early the lower string and/or the crossover may be damaged by the torque.